1. Field of the Invention
The present invention relates to a startup device, particularly to a high-voltage (HV) startup device.
2. Description of the Related Art
Numerous techniques have been used to control switching power supplies. One of the most common is know as pulse width modulation (PWM) in which the switching frequency is held constant while the duty cycle is modulated to control the output. Another common technique is known as pulse frequency modulation (PFM) in which the switch on-time or off-time is held constant, and the frequency is modulated to control the output. In another technique known as hysteretic control (or ripple regulation) frequency and duty cycle are varied so as to keep the output ripple constant.
As shown in FIG. 1 and FIG. 2, a switching power supply of conventional technology includes a rectifier 10, a pulse width modulator (PWM) 12, a switch 14 and a transformer 16 having a primary winding PW and a secondary winding SW and the switch 14 connected to the primary winding PW. The secondary winding SW is connected with a load 18. The rectifier 10 receives an alternating current (AC) signal, and rectifies it to output a direct current (DC) voltage VH applied to the primary winding PW. The PWM 12 receives a high voltage from the rectifier 10 to output a pulse width modulation (PWM) signal to the switch 14. In other words, the high voltage is used to trigger the PWM 12 to output the PWM signal. A voltage VG is periodically applied (TON) to the gate of the switch 14 to control the transfer of power from the primary winding PW to the secondary winding SW. When the switch 14 is turned on, the energy is stored in the transformer 16. As the switch 14 is turned off, the stored energy in the transformer 16 is discharged. However, after sending out the PWM signal, the PWM 12 still receives the high voltage from the rectifier 10 to operate, which results in energy loss.
To overcome the abovementioned problems, the present invention provides a high-voltage (HV) startup device, so as to solve the afore-mentioned problems of the prior art.